The brain contains glial cells, which out number neurons by ten-fold. Long considered to provide only a passive role, increasing evidence now suggests that specialized glial cells, called astrocytes, actively participate in normal brain function through interactions with neurons1. Moreover, astrocyte-neuron interactions are involved in epileptic seizure activity. This is of importance because the mechanisms that give rise to epilepsy are not understood;despite the fact that there are three million epilepsy patients in North America for whom a greater understanding of the mechanisms involved is the only rational way to provide improved disease treatment. Such progress is hampered by the fact that many fundamental and important aspects of astrocyte-neuron interactions remain unclear or experimentally unexplored. In this proposal we seek to determine if astrocytes have an intracellular calcium excitability code, and how this impacts synaptic responses to nearby neurons. We will test the hypothesis that kinetically distinct intracellular calcium waveforms within astrocytes trigger distinct forms of exocytosis, and affect nearby neurons in separable and stereotyped ways. Taken together the results of these experiments will establish the logic of astrocyte-neuron communication, and the basis of signaling between these brain cell types. We will thus discover the mechanisms that go awry in epilepsy. We have three specific aims, Specific Aim 1. Tests the hypothesis that physiological patterns of intracellular calcium transients select between modes of astrocyte exocytosis. Specific Aim 2. Tests the hypothesis that distinct modes of astrocyte exocytosis impact neurons in separable ways. Specific Aim 3. Tests the hypothesis that astrocyte exocytosis affects short and long term changes in synaptic strength. PUBLIC HEALTH RELEVANCE We will determine whether astrocytes undergo exocytosis during intracellular calcium transients, as well as determine the effects on neuronal synaptic transmission and plasticity. In so doing we will establish the basis for understanding the active roles of astrocytes within neuronal networks in general, with specific implications for in vitro models of epileptic seizures. This is important because the mechanisms that give rise to epileptic seizures are incompletely understood, and there is an unmet need for the clinical management of epilepsy.